Description

The malloc() and free() functions provide a simple general-purpose memory allocation package
that is suitable for use in high performance multithreaded applications. The suggested
use of this library is in multithreaded applications; it can be
used for single threaded applications, but there is no advantage in doing
so. This library cannot be dynamically loaded with dlopen(3C) during runtime because
there must be only one manager of the process heap.

The malloc() function returns a pointer to a block of at least
size bytes suitably aligned for any use.

The argument to free() is a pointer to a block previously allocated
by malloc() or realloc(). After free() is performed this space is available
for further allocation. If ptr is a null pointer, no action occurs.
The free() function does not set errno.

Undefined results will occur if the space assigned by malloc() is overrun
or if a random number is handed to free(). A freed pointer
that is passed to free() will send a SIGABRT signal to
the calling process. This behavior is controlled by mallocctl().

The memalign() function allocates size bytes on a specified alignment boundary and
returns a pointer to the allocated block. The value of the returned
address is guaranteed to be an even multiple of alignment. Note that
the value of alignment must be a power of two, and must be
greater than or equal to the size of a word.

The realloc() function changes the size of the block pointed to by
ptr to size bytes and returns a pointer to the (possibly moved)
block. The contents will be unchanged up to the lesser of the
new and old sizes. If the new size of the block requires movement
of the block, the space for the previous instantiation of the block
is freed. If the new size is larger, the contents of the
newly allocated portion of the block are unspecified. If ptr is NULL,
realloc() behaves like malloc() for the specified size. If size is 0
and ptr is not a null pointer, the space pointed to is
freed.

The valloc() function has the same effect as malloc(), except that the
allocated memory will be aligned to a multiple of the value returned
by sysconf(_SC_PAGESIZE).

After possible pointer coercion, each allocation routine returns a pointer to
a space that is suitably aligned for storage of any type
of object.

The malloc(), realloc(), memalign(), and valloc() functions will fail if there is
not enough available memory.

The mallocctl() function controls the behavior of the malloc library. The options
fall into two general classes, debugging options and performance options.

Writes misaligned data into the buffer after free(). When the buffer is reallocated, the contents are verified to ensure that there was no access to the buffer after the free. If the buffer has been dirtied, a SIGABRT signal is delivered to the process. Setting value to 1 means yes and 0 means no. The default behavior is to not write misaligned data. The pattern used is 0xdeadbeef. Use of this option results in a performance penalty.

MTINITBUFFER

Writes misaligned data into the newly allocated buffer. This option is useful for detecting some accesses before initialization. Setting value to 1 means yes and 0 means no. The default behavior is to not write misaligned data to the newly allocated buffer. The pattern used is 0xbaddcafe. Use of this option results in a performance penalty.

MTCHUNKSIZE

This option changes the size of allocated memory when a pool has exhausted all available memory in the buffer. Increasing this value allocates more memory for the application. A substantial performance gain can occur because the library makes fewer calls to the OS for more memory. Acceptable number values are between 9 and 256. The default value is 9 for 32–bit code and 64 for 64–bit code. This value is multiplied by 8192.

MTEXCLUSIVE

By default, libmtmalloc allocates 2*NCPUS buckets from which allocations occur. Threads share buckets based on their thread ID. If MTEXCLUSIVE is invoked, then 4*NCPUS buckets are used. Threads with thread IDless than 2*NCPUS receive an exclusive bucket and thus do not need to use locks. Allocation performance for these buckets may be dramatically increased. One enabled, MTEXCLUSIVE cannot be disabled. This feature can also be enabled by setting the option MTEXCLUSIVE in the environment variable MTMALLOC_OPTIONS. See libmtmalloc(3LIB).

MTREALFREE

This option sets the threshold for calling madvise(3C) with MADV_FREE. Calling madvise() will result in the memory associated with the allocation being returned to the kernel. When freed, allocations greater than value*pagesize will have madvise() called. If value is less than 2, it will be set to 2.

Return Values

If there is no available memory, malloc(), realloc(), memalign(), and valloc() return
a null pointer. When realloc() is called with size > 0 and
returns NULL, the block pointed to by ptr is left intact. If
size, nelem, or elsize is 0, either a null pointer or a
unique pointer that can be passed to free() is returned.

If malloc() or realloc() returns unsuccessfully, errno will be set to indicate
the error.

Errors

The malloc() and realloc() functions will fail if:

ENOMEM

The physical limits of the system are exceeded by size bytes of memory which cannot be allocated.

EAGAIN

There is not enough memory available to allocate size bytes of memory; but the application could try again later.

Usage

Comparative features of the various allocation libraries can be found in the
umem_alloc(3MALLOC) manual page.